Tools: add unified device tree overlay to load all requisites

no more loading multiple capes to enable pru, spi, pwm etc. Steps to load the cape during boot also added.
Tools: Remove unrequired pru0 pin setups from BB-PXF-01-00A0 overlay
GCS_MAVLink: ignore alignment warnings in MAVLink headers
Airspeed: fixed example to setup pin
Replay: fixed build
Build: enable px4flow driver on Pixhawk
OptFlow: rename and restructure OpticalFlow class

AP_OpticalFlow.h becomes simply a file that includes all other optical
flow header files.
OpticalFlow class simplified to only return surface quality, raw output
and velocity vector.
OptFlow: reorganise ADNS3080 to simplified interface
OptFlow: add PX4Flow support
PX4: start px4flow driver
Copter: enable OPTFLOW - do not merge to master
Copter: optflow parameters moved to optflow class
Copter: simpler optflow logging

Removed of_loiter control output from message
Copter: instantiate optflow on Pixhawk

Run sensor reads from scheduler
Copter: integrate optflow_enable param move to optflow class
Copter: Of_Loiter uses sensor velocity instead of integrated position
Copter: OF log fix to save vel as float
Copter: add TimeMS to OF dataflash log msg
OptFlow: add ground_distance_m
OptFlowPX4: retrieve ground distance from sensor